This invention relates to external combustion heat engines, heat pumps, and refrigeration devices.
Experimenting with Stirling engines using air at mean atmospheric pressure as a working fluid, it was found that a main factor limiting power for a given size of engine was the time required for heat to transfer to the working fluid. For this reason an experimental engine that was not a Stirling engine was built to take advantage of and actually use this time required for heat transfer which is referred to as thermal lag. An ultimately simple engine using only a cooled cylinder and piston connected to a heated chamber ran both as a free piston engine and as a kinematic engine.